Experimental investigation of the effective parameters in design of non-phase hydraulic jet in reduction of 90 degrees bend scour

Absract: Some events such as scour and deposition in rivers may endanger the stability of the outer bank’s bend and reduce the navigable width of them. So far, different techniques have been developed to control this phenomena such as bottom vanes, submerged groynes, bandal-like structures. In the present study, the influences of non-phase hydraulic jet usage on bed scouring and deposition in a 90 bend has been investigated experimentally. Experiments were conducted in a experimental channel to measure the variations of bed topography under a clear water condition. The90 bend is connected to an upstream straight reach 5 m long and a downstream straight reach 3 m long. The channel was rectangular having 70c m width and 280cm radius of bend to centerline. (R/B=4; where R=radius and B=flume width). Four Froude number values including 0.37, 0.41, 0.45 and 0.47 with a constant flow depth of 11cm under clear water condition were used for each test. Bed topography was measured with a laser meter and contour lines were plotted with the Tec plot360software. The scour geometry in a bend depends on channel geometry (channel width, channel , radius and bed slope), flow conditions (depth and discharge or velocity), non-phase hydraulic jet characteristics (length, angle with bank, location in bend), sediment properties (specific gravity, grain size), and fluid parameters (density and viscosity). Therefore, by Using the Buckingham theory and after eliminated the parameters with constant values, the important parameters were q/Q, D/h , Fr and α.Three-dimensional (3D) velocity components were measured using the electromagnetic velocity meter JFE ALEC model ACM3-RS in tests with and without an installed non-phase hydraulic jet. The sampling rate was 20 Hz and time of sampling was 60 seconds. So the minimum 1200 data were collected for each point and their mean was used for determine the 3D flow pattern. The results showed that with installing non-phase hydraulic jet, the maximum scour depth is reduced as much as 77%, 82.7% for Froude numbers equal to 0.37 and 0.41, respectively. It was also found that generally the scour depth occurs away from the outer bank and it was shifted toward the middle parts of the section in the main experiments, which can positively result in the increase in the navigable width and also sediment deposition was observed at the outer bank in some experiment. The results show that non-phase hydraulic jet significantly modified bed topography and reduced the maximum scour depth in outer bank. In this paper to evaluate the effect of space between porous tube and outer bank , three different space (D=0,2.5 and 8 cm) were used. It was found that by decreasing the space between outer bank and porous tube the amount of scour decreases and the maximum scour depth reduced as much as 87.9%, 54.5% and 15.15% for space equal to 0, 2.5 and 8 cm, respectively and it is most efficient when it placed on outer bank. Maximum scour was usually saw near the outer bank, and attributed to the maximum stream wise velocities that occur near the toe of the bank , also the analysis of data have shown that the presence of non-phase hydraulic jet caused uniformity in the velocity distribution at upstream and the high velocity zone moved toward the center of the channel and inner wall. The installing a non-phase hydraulic jet on the outer bank of bend, by its rising velocities, generated a secondary flow that rotates in the sense opposite to the curvature-induced secondary flow. The jet-induced secondary flow cell causes an inwards shift of the cores of maximum streamwise velocity and maximum vertical velocity impinging on the bed, that both of them play an important role in the cour in the outer bank.